1. Field of the Invention
The present invention relates to an output processing circuit for a radio frequency receiver, which is incorporated in an AM/FM RF receiver, particularly, a vehicle FM stereo receiver to perform demodulation output processing such as stereo separation characteristic control and control for removing high- and low-frequency components.
2. Description of the Prior Art
In a vehicle AM or FM stereo receiver, the intensity of a reception electric field is not necessarily constant. Therefore, proper implementations must be provided to maintain good tone quality even if the intensity of the electric field is changed. For example, a detector output (signal meter output) having a level corresponding to the intensity of the electric field is utilized to control stereo separation characteristics and high- and low-frequency characteristics in the range from a low intensity to an intermediate intensity of the electric field in accordance with the present intensity of the electric field, thereby obtaining comfortable tone quality.
FIG. 1 is a block diagram showing a conventional arrangement of an FM demodulation output processing circuit to improve tone quality in the range of the low intensity to the intermediate intensity of the electric field.
Signal meter output E61 corresponding to the intensity of an electric field and generated by FM internal frequency detector (FM-IF detector) 61 is supplied through potential slider 62 to demodulation output control circuit 63 including a blender for controlling right- and left-channel separation of a stereo signal.
Potential slider 62 includes a time constant circuit comprising semivariable resistor 64, constant voltage element 65 having a diode or an LED, resistor 66, and capacitor 67. The DC level of signal meter output E61 is shifted or slid by a predetermined amount. The gradient of signal meter output E61 with respect to the electric field intensity is controlled by semivariable resistor 64.
Assume that the level of signal meter output E61 from FM-IF detector 61 is changed to characteristic curve A in FIG. 2 in accordance with the intensity of the electric field. In this case, the value of constant voltage element 65 in potential slider 62, i.e., a level slide or shift amount is preset to be, e.g., 2 V, and the gradient controlled by semivariable resistor 64 with respect to the intensity of the electric field is present to be 2 V/40 dB.mu.. In this case, output E62 from potential slider 62 is represented by characteristic curve B shown in FIG. 2. A cross-talk characteristic curve of the blender in modulation output control circuit 63 which receives output E62 from potential slider 62 is represented by curve C in FIG. 2.
FIG. 3 is a graph showing the relationship between output control signal E62 from potential slider 62 and the cross-talk component. This relationship is extracted from the graph in FIG. 2. When this cross-talk characteristic curve is obtained, right and left-channel separation can be controlled in proportion to the intensity of the electric field in the range from the low intensity to the intermediate intensity as the cross-talk component is decreased with a higher intensity of the electric field. The amount of noise in the stereo-separated audio signal is in inverse proportion to the cross-talk component. Therefore, best tone quality with suppressed noise can be always obtained. When modulation output control circuit 63 comprises a high cut controller for cutting off a high-frequency component and a low cut controller for cutting off a low-frequency component in place of the blender, the high- and low-frequency components can be cut off in accordance with the present intensity of the electric field.
The operation range (D in FIG. 2) of demodulation output control circuit 63 must be flexibly changed in correspondence with FM receiver markets. However, in the FM demodulation output control circuit in FIG. 1, the voltage value of constant voltage element 65 is fixed, and level shift amounts (i.e., slide amounts) can be set to be only discrete values. For this reason, the values defining operation range D in FIG. 2 are also discrete values. For example, when one Si diode is used as constant voltage element 65, the level shift amount is set to be about 0.7 V. When two series-connected Si diodes are used, the level shift amount is set to be about 1.4 V. When an LED is used as constant voltage element 65, the level shift amount is set to be about 1.7 V to 2 V. As a result, the operation range of the conventional demodulation output control circuit cannot be flexibly changed in correspondence with the respective markets. In this manner, in a conventional arrangement, the operation range of demodulation output control of the RF receiver cannot be continuously changed.